Sclera, Cornea, and Uvea

The eyeball consists of three tunics or layers:

• Sclera (tunica fibrosa), is opaque white made of dense connective tissue. It is relatively avascular, which has tendons of the six extrinsic muscles of the eye attached to its outer surface. Its inner side faces the choroid, from which it is separated by a layer of loose connective tissue and an elastic tissue network known as the suprachoroid lamina.
• • The episclera is an external layer of the sclera connected by loose connective tissue with the Tenon’s capsule. It separates the eyeball from adipose tissue in the orbit.
• Cornea (also part of tunica fibrosa) is transparent, lacks blood vessels, and is extremely rich in nerve endings. The anterior surface of the cornea is always kept wet with a film of tears retained by microvilli of the apical epithelial cells. There are five layers:
• Corneal epithelium is stratified squamous and consists of five to seven layers of cells. Cells of the outer surface have microvilli. All the cells are connected to one another by desmosomes. The epithelium of the cornea is very sensitive, contains a large number of free nerve endings, and has a remarkable wound healing capacity. At the limbus, the corneoscleral junction, the corneal epithelium is continuous with that of the conjunctiva.
• Layer of Bowman is 6-9μm thick, consists of type I collagen fibrils, and lacks elastic fibers. This layer is transparent. It acts as a protective barrier to trauma and invasion.
• Stroma or substantia propria represents about 90% of the thickness of the cornea. Bundles of type I and V collagen form thin layers. Fibers and layers are separated by an extracellular matrix rich in proteoglycans containing chondroitin and keratan sulfate.
• Membrane of Descemet, one of the thickest basement membranes in the body (5-10μm thick), is produced by the corneal endothelium and contains type VII collagen, which forms a hexagonal array of fibers.
• Corneal endothelium lines the posterior surface of Descemet's membrane and faces the anterior chamber of the eye. It consists of a single layer of squamous epithelial cells, with impermeable intercellular spaces preventing influx of aqueous humor into the corneal stroma. All metabolic exchanges of the cornea occur across the endothelium.
The external stratified squamous epithelium is nonkeratinized and densely supplied with sensory-free nerve endings that trigger the blinking reflex. The stroma comprises approximately 90% of the cornea’s thickness, consisting of some 60 layers of long type I collagen fibers arranged in a precise orthogonal array and alternating with flattened fibroblasts. The stroma is lined internally by corneal endothelium. The corneoscleral junction or limbus is a transition area. The corneal epithelium changes to conjunctiva. Bowman’s layer ends here. It is highly vascularized. Endothelium-lined channels create the canal of Schlemm (sinus venosus sclerae; drains fluid from the anterior chamber of the eye).
• Uvea (tunica vasculosa) forms the pigmented vascularized tunic of the eye and is divided into three regions:
• The choroid consists of three layers, being Bruch's membrane, the choriocapillaris, and the choroidal stroma. The Bruch’s membrane is the innermost component of the choroid, consisting of a network of collagen and elastic fibers and basal lamina material. Basal lamina derives from the pigmented epithelium of the retina and the endothelium of the underlying fenestrated capillaries. The choriocapillaris contains fenestrated capillaries that supply oxygen and nutrients to the outer layers of the retina and the fovea. The choroidal stroma consists of large arteries and veins surrounded by collagen and elastic fibers, fibroblasts, a few smooth muscle cells, neurons of the autonomic nervous system, and melanocytes.
• The ciliary body is anterior to the ora serrata and represents the ventral projection of both the choroid and the retina. It is made up of the uveal portion and the neuroepithelial portion. The uveal portion includes the continuation of the outer layer of the choroid, known as the supraciliaris. The ciliary muscle, a ring of smooth muscle that regulates tension on the suspensory ligaments of the lens (the ciliary zonule). A layer of fenestrated capillaries supplying blood to the ciliary muscle. The neuroepithelial portion includes an outer pigmented epithelial layer, continuous with the retinal pigmented epithelium. The pigmented epithelial layer is supported by a basal lamina continuous with Bruch's membrane. It also includes an inner nonpigmented epithelial layer.
• The iris is a continuation of the ciliary body and is located in front of the lens. Pupil is the central aperture of this thin disc. At this position, it forms a gate for the flow of aqueous humor between the anterior and posterior chambers of the eye and also controls the amount of light entering the eye. The iris has two components, which are the anterior uveal or stromal face and the posterior neuroepithelial surface. The anterior (outer) uveal face is of mesenchymal origin and has an irregular surface. It is formed by fibroblasts and pigmented melanocytes embedded in an extracellular matrix. The number of pigmented melanocytes and the amount of pigment are major factors in determining the colour of the iris. The posterior (inner) neuroepithelial surface consists of two layers of pigmented epithelium. The outer layer, a continuation of the pigmented layer of the ciliary epithelium. The inner layer consists of myoepithelial cells that become the dilator pupillae muscle.